The Sun in Visible
Light

White (unfiltered) light

Calcium-K (filtered) light

Hydrogen-alpha (filtered) light

What we call "visible light" can be broken into two categories: white
light, and filtered light. White light pictures show how the Sun appears
to the naked eye, when all the colors of the rainbow are collected by the
camera. Filtered light refers to the selection of specific colors by
using a filter. This is sometimes done to reveal features of the Sun that
get lost in white light pictures. For more discussion, including a
hands-on activity, check out the "Image Filtering" lesson in the YPOP
Solar Classroom.

In a white light photo, the part of the Sun that we see is called the
"photosphere". It has a temperature of about 6000 degrees
Celsius, much cooler than the corona. Sometimes there are notable dark
spots. These "sunspots" come and go, so look at the Sun again in a few
days to see how its appearance has changed.

A filtered image often can show features on the Sun that are harder to
see in the glare of unfiltered light. Two common filters for observing the
Sun are the "Hydrogen-alpha" and the "Calcium-K" filters. They allow light
that is within the visible part of the spectrum, but is tough to see
without filters. H-alpha filters allow the camera to see only red light
with a wavelength of 656.3 nanometers; and Calcium-K filters allow a
camera to see only blue light with a wavelength of 393.4 nanometers. These
particular colors of sunlight come not from the Sun's photosphere, but
from the next highest layer, called the
"chromosphere." We normally can't see the chromosphere in white light
pictures, because it is so much fainter than the photosphere that the
photosphere "washes it out." It's like trying to see a candle held in
front of a much brighter spotlight. The temperature of the chromosphere
varies between 6000 and 20000 degrees Celsius. Sometimes long, thin, dark
ribbon-like structures can be seen -- these are called
"filaments" when they're seen on the disk part of the Sun, and
"prominences" when they're seen extending beyond the edge of the
limb.

The white-light and Calcium-K images comes from Big Bear Solar
Observatory in
California, which is operated by the New Jersey Institute of Technology.
The H-alpha image comes from Learmonth
Solar Observatory in Western Australia.